1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to a liquid crystal display device including a photosensor for sensing brightness of external light such that the photosensor can stably sense the external light without influence of a temperature or external noise, and a method for automatically controlling brightness.
2. Discussion of the Related Art
As the information age has arrived, a display field for visually displaying an electrical information signal has been rapidly developed. Accordingly, various flat display devices with excellent performance such as slimness, light weight and low power consumption have been rapidly developed and used as a replacement for Cathode Ray Tubes (CRTs).
Examples of such flat display devices include a Liquid Crystal Display (LCD) device, a Plasma Display Panel (PDP) device, a Field Emission Display (FED) device, an Electroluminescent Display (ELD) device, and the like, each of which includes a flat display panel for displaying an image as an essential component. The flat display panel includes a pair of transparent insulating substrates which are adhered to each other with a light emitting layer or a polarized material layer interposed therebetween.
Among such devices, the LCD device controls light transmission of liquid crystal using an electric field so as to display an image. An image display device includes a display panel having liquid crystal cells, a backlight unit for irradiating light to the display panel, and a driving circuit for driving the liquid crystal cells.
In the field of the LCD device, an Active Matrix LCD (AMLCD) is mainly used. In the AMLCD, one Thin Film Transistor (TFT) defines one pixel, and one TFT controls the voltage level of the pixel as a switching element so as to change the light transmission of the pixel, thereby displaying an image.
A general LCD device includes a liquid crystal panel including a plurality of TFTs arranged in a matrix to display an image, a gate driver for controlling the input of a data signal to the liquid crystal panel, a data driver for outputting a data signal to the liquid crystal panel, a timing controller for controlling the timing of each of the drivers, and a backlight unit for irradiating light to the liquid crystal panel. The backlight unit includes a backlight element located on a rear surface of the liquid crystal panel to visually display the data signal and a control unit for controlling the backlight element. Each of such components receives driving power from a power supply unit supplying power suitable for driving each of the components, and the control unit is integrated on a Printed Circuit Board (PCB).
Although not shown, the backlight element may be one or more fluorescent lamps or a plurality of Light Emitting Diodes (LEDs).
In the LCD device having such a configuration, hydrogenated amorphous silicon (a-Si:H; hereinafter, referred to as amorphous silicon) is mainly used as a semiconductor layer of a TFT. This is because a semiconductor layer can be manufactured with a large area and high productivity and can be deposited at a low substrate temperature of 350° C. or less and a cheap insulating substrate can be used.
However, the characteristics of amorphous silicon may deteriorate by the irradiation of light. In addition, it is difficult to use amorphous silicon in a driving circuit due to the electrical characteristics of the TFT (low field effect mobility: 0.1 to 1.0 cm/V·s) and deterioration in reliability.
Therefore, in an amorphous silicon TFT substrate, an insulating substrate and a PCB are connected using a Tape Carrier Package (TCP) driving Integrated Circuit (IC). Thus, cost required for the driving IC occupies a large part of production cost.
Meanwhile, the liquid crystal panel is formed by adhering two substrates to each other and is divided into a display area in which TFTs are formed so as to display an image and a non-display area in which drivers and signal wires are formed.
In more detail, in the display area, a plurality of gate lines and data lines are arranged in a matrix so as to cross each other, and the TFTs are formed at intersections where the gate lines and the data lines cross each other.
The gate driver and the data driver receive a scanning signal and a data signal from an external device and control the TFTs of the display area through the gate lines and the data lines, thereby changing the light transmission of the liquid crystal.
Although not shown, a timing controller and a power supply unit are mounted on an independently mounted PCB so as to be connected to the gate driver and the data driver. The backlight unit is mounted on the rear surface of the liquid crystal panel.
In the LCD device using amorphous silicon or polysilicon, since light generated by the backlight unit is constant, there is a limitation to control the brightness of the LCD device according to the brightness of external light. Accordingly, in order to control the brightness of the liquid crystal display device, an input voltage of the backlight unit attempts to be controlled or brightness of external light attempts to be measured using a photosensor or the like and be fed back so as to control the brightness of the backlight lamp of the LCD device.
Hereinafter, a conventional LCD device will be described with reference to the accompanying drawings.
FIG. 1 is a diagram showing a conventional mobile phone including an external photosensor.
FIG. 1 shows an application of the conventional LCD device, that is, a mobile phone 1. The mobile phone includes a screen 5 corresponding to the liquid crystal panel and an external unit having a keypad 20 for enabling a user to control an image displayed on the screen 5.
An external light sensor 10 for receiving and sensing external light is further included in the keypad 20.
That is, recently, in a mobile or small and medium-sized model, a sensor for sensing external light is mounted in an external unit, independent of a liquid crystal panel. Alternatively, in a mobile phone, an external light sensor may be further included in a camera unit.
In this case, the external light sensor 10 is formed on a semiconductor wafer as an independent sensor and is included in an independent module, independent of the liquid crystal panel.
The conventional LCD device has the following problems.
In order to control the brightness of the backlight unit, the external light sensor for sensing the external light is included in the external unit of the LCD device as the independent module, independent of the liquid crystal panel. In this case, the independent module is necessary for operating the external light sensor, thereby increasing the price of the device. In addition, a process of mounting the independent module is required and thus assembling the independent module in the LCD device becomes complicated.